FIG. 10 is a block diagram showing an illustrative circuit configuration of a receiver employing a digital AGC (Automatic Gain Control) system, and illustrating a receiver used in a radio base station. In the drawing, reference numeral 100 means an antenna, 200 is a receiver for use in a frequency band of RF to IF, 210 is a variable attenuator, 220 is an AGC amplifier, 310 is a quadrature detector, 320 is an A/D converter, 330 is a D/A converter, 340 is a control signal line for the variable attenuator, and 400 is a digital signal processor (DSP).
FIG. 11 is a flowchart showing an algorithm in the conventional AGC system.
Referring to FIG. 10, a first description will be given of the operation of the circuits in the receiver.
The antenna 100 receives a time division multiple access (hereinafter abbreviated as TDMA) RF signal transmitted from a mobile unit terminal. The receiver 200 performs down conversion of a signal in the RF band into a signal in the IF band.
In addition to the down conversion function, the receiver 200 has the AGC function of keeping through a gain control device a receive signal level at a constant level at which amplitude information is not lost. The AGC function is carried out by attenuation control by the variable attenuator 210 and amplification factor control by the AGC amplifier 220.
The attenuation control by the variable attenuator 210 includes control in steps of several decibels according to digital control in steps of several bits, ON/OFF control in steps of tens decibels, and so forth. A description will now be given of a case where the latter ON/OFF control in steps of tens decibels is used.
On the other hand, the AGC amplifier 220 can perform a successive gain control according to voltage.
The signal in the IF band outputted from the receiver 200 is inputted into the quadrature detector 310.
In the quadrature detector 310, an IF band modulated signal outputted from the receiver 200 is detected by a locked oscillator with substantially the same frequency as the IF band frequency, thereby outputting a baseband signal including an I component (in-phase component) and a Q component (quadrature component).
The baseband signal including the I component and the Q component outputted from the quadrature detector 310 is inputted into the A/D converter 320. For example, fourfold oversampling is made for each symbol in the signal including the I component or the Q component to output a several-bit quantized signal for each sample.
The quantized signal outputted from the A/D converter 320 is inputted into the digital signal processor 400 to execute the AGC algorithm as shown in FIG. 11.
Next, a description will now be given of the AGC algorithm in the prior art with reference to FIG. 11.
The digital signal processor 400 receives a receive start timing flag (Step 900). The receive start timing flag is known in the radio base station, and is transmitted for each burst signal with a receive timing shift of .+-.0 symbol from a mobile station.
The digital signal processor 400 receives the receive start timing flag to set a control value about a receive signal level derived from a receive burst signal in the same time slot in the previous frame for the gain control devices 210 and 220 through the control signal line 340 and the D/A converter 330. In such a manner, the ATT 210 and the AGC amplifier 220 serving as the gain control device are controlled to provide the same gain as that in the same time slot in the previous frame. The receiver 200 receives a burst signal in the same time slot in a current frame according to the set gain (Step 901).
The receive burst signal in the same time slot in the current frame is converted into the baseband signal including the I component and the Q component in the quadrature detector 310. For example, the fourfold oversampling is made to the baseband signal including the I component and the Q component in the A/D converter 320. The resultant Y samples are selected by the digital signal processor 400 (Step 902) to derive a control error to the gain control device setting made to the current receive burst signal (Step 903). The "control error" as used herein corresponds to a difference between a receive level and a desired level, and means a width to be converged by the gain control device.
Subsequently, it is decided whether or not the above control error can be followed by only the AGC amplifier (Step 904). When the control error can be followed by only the AGC amplifier, a fine adjustment is made to a gain corresponding to the above control error when a burst signal in the same time slot in the next frame is received (Step 907). Further, when the control error can not be followed by only the AGC amplifier, a rough adjustment is made to invert a setting of the variable attenuator when the burst signal in the same time slot in the next frame is received (Step 905).
In such a manner, it is determined which processing should be made to the burst signal in the same time slot in the next frame, thereafter waiting reception of a burst signal in the same time slot in the above next frame (Step 906).
As set forth above, when the conventional AGC system is used, the control is made to the receive burst signal in the same time slot in the next frame depending upon the control error of the receive burst signal in the same time slot in the previous frame. However, in a burst signal transmitted from the mobile station at a time of, for example, call, or channel switching, transmitting power is irregular due to a variation in distance between the mobile station and the radio base station, and so forth. Further, the burst signal is not always periodically transmitted for each frame, resulting in the aperiodic burst signal. Therefore, in the radio base station receiving such a burst signal, the previous frame can not used so that a receive level must be converged by the AGC to a desired value in a received burst signal in a current frame.
On the other hand, even if the conventional AGC system is applied to a periodic burst signal, only the control error to the receive burst signal can be used as information for the AGC, and the AGC system includes no means for detecting transmitting power control according to interference or silence at the mobile station. As a result, there is a problem in that an erroneous control may be made in the gain control.
Further, irrespective of the periodic burst signal or the aperiodic burst signal, there is another problem of an erroneous gain control when the receive timing is shifted.